1. Field of the Invention
The present invention relates to optical waveguide sensors utilizing the optical mode coupling, measuring apparatus using the optical waveguide sensor and measuring method using a sensor.
2. Description of the Related Art
In recent years, technologies for detecting and quantitating chemical substances, such as ammonium ion or sodium ion, or biological substances, such as DNA or antigen antibody, have been growing in importance in the areas of medical care, health, food, development of new drugs, chemistry and biochemistry. In particular, a number of proposals have been made of optical waveguide sensors capable of detecting and quantitating ultramicro amounts of substance with high sensitivity, high speed and simplicity by making use of the interaction between substance present on an optical waveguide and lightwaves (see, for example, Reference (1) or Reference (2) in the following Related Art List).
Related Art List
    (1) Japanese Patent Application Laid-Open No. Hei8-75639.    (2) Japanese Patent Application Laid-Open No. 2002-148187.
These optical waveguide sensors operate, using the change in absorptance of lightwaves caused by the acquisition of substance to be detected or the change in surface plasmon resonance state caused by the acquisition of substance to be detected.
The former type of optical waveguide sensor is comprised of a cladding layer made of light transmitting material and a single line of core layer whose refractive index is larger than that of the cladding layer. The cladding layer has a function of confining light by enclosing part of the core layer, and the core layer has a function of a waveguide through which lightwaves propagate while undergoing total reflection. The part of the core layer which is not enclosed by the cladding layer is in contact with liquid or gas containing the substance to be detected. Of the lightwaves led into the core layer, only the lightwaves having the angle of incidence and wave number that satisfy an eigenvalue equation propagate while undergoing total reflection. At this time, the evanescent waves extend about the length of one wave outside the core layer. If there is any change in the mass of the substance to be detected within the range of the evanescent waves, there will be a change in the absorbed amount of lightwaves of a specific wavelength corresponding to the substance. With white light (multi-wavelength light) used as incident wave, the type and amount of detected substance are known from the spectrum distribution of the output waves.
The latter type of optical waveguide sensor is an optical waveguide type surface plasmon resonance sensor wherein a metal thin film with receptors attached thereon is so disposed on the core layer that it comes in contact with liquid or gas containing the substance to be detected. The lightwaves propagated through the core layer, which are incident on the lower surface of the metal thin layer under total reflection conditions, ooze out once to the upper surface of the conductive thin film and become the evanescent waves. At this point, when the wave number of the evanescent waves originating in p polarization agrees with that of the surface plasmon, a resonance occurs, thus transforming the incident wave energy into the resonance energy and nearly zeroing the reflected wave energy. This is called a surface plasmon resonance.
Here, the wave number of surface plasmon is defined by the complex dielectric constant (or complex refractive index, and hereinafter “dielectric constant” and “refractive index” will be used as equivalents) of conducting thin film, the dielectric constant of a reaction area, and the wavelength of the surface plasmon. The wave number of the evanescent waves is defined by the dielectric constant of the core layer and the wavelength and the angle of incidence of the incident waves.
The lightwaves, which are white light (multi-wavelength light), contain diverse wavelengths and-diverse angles of incidence corresponding thereto. Only the energy of lightwaves that have the wavelength and angle of incidence satisfying the resonance conditions is consumed as the resonance energy, so that it is possible to identify the type and amount of detected substance to be detected from the spectrum distribution of the output waves.
Known technologies, however, have inherent problems. That is, the lightwaves to be propagated through the core layer must be white light (multi-wavelength light) in order to determine the type and amount of substance to be detected from a spectrum. This results in a large source of light and measuring equipment.
Moreover, the length of the waveguide must be long if the sensitivity is to be raised. This results in a large-size detector.
Moreover, the energy absorbed of the incident wave energy is very small, and the necessity for measuring a small change in a large signal results in the problem of low accuracy of measurement.